The present invention relates to a softbit speech decoder, and more specifically, to a softbit speech decoder capable of executing when the received signal contains no speech data.
In digital speech communication, bit errors in noisy channels cause a reduction in quality. To help eliminate bit errors that still remain, a process called error concealment is performed.
In the article entitled “Softbit Speech Decoding: A New Approach to Error Concealment” by Tim Fingscheidt and Peter Vary, published in the “IEEE Transactions on Speech and Audio Processing”, Vol. 9, No. 3, March 2001, the authors describe an error concealment approach using a softbit speech decoder. This article is herein incorporated by reference in its entirety.
Please refer to FIG. 1. FIG. 1 is a functional block diagram of a speech decoding system 10 utilizing a softbit speech decoder 50 according to the related art. In the speech decoding system 10, an equivalent channel 12 provides speech parameters X(t) and bit error probability data P(t) to a parameter transition probability circuit 14 of the softbit speech decoder 50. The parameter transition probability circuit 14 takes this data and thereby calculates speech-parameter error probability data W(t), which is sent to an a posteriori probability circuit 16. The a posteriori probability circuit 16 receives prior knowledge from an a priori knowledge circuit 18 and thereby provides probability information about any possibly transmitted bit combination. All of this data is output from the a posteriori probability circuit 16 to a parameter estimation circuit 20, which generates estimated speech parameters V(t) accordingly.
As long as the equivalent channel 12 is receiving speech data, the equivalent channel 12 provides this speech data to the softbit speech decoder 50, and the softbit speech decoder 50 will operate normally. However, if the equivalent channel 12 receives control signals instead of speech data, the parameter transition probability circuit 14 will not be able to produce the speech-parameter error probability data W(t), and the softbit speech decoder 50 will cease operation. Therefore, the softbit speech decoder 50 will not be able to reduce the subjective effects of bit errors that have not been eliminated by channel decoding when no speech data are transmitted.